In recent years, there have been active researches and developments made for an optical circuit having, as a major constitutional component, a silicon-based wire optical waveguide using single crystal silicon, amorphous silicon, or a silicon compound material, such as silicon oxide, silicon nitride and silicon oxynitride, as a core of the optical waveguide.
This is because a large difference in specific refractive index is obtained between a silicon-based core material and a quartz-based clad material, and therefore even when the optical waveguide is bending to a small curve radius, the radiation loss of light may be prevented, thereby achieving a considerable miniaturization an optical circuit. Furthermore, the production process of a silicon CMOS LSI may be applied thereto, and therefore reduction in production cost due to mass production is being expected.
In general, an optical circuit having a silicon-based wire optical waveguide as a major constitutional component is formed in a single plane due to the manufacturing process reasons, and it is the most ordinary practice that input and output of light to the optical circuit is performed through the cross section of the optical waveguide in the direction that is perpendicular to the cross section of the optical waveguide within the same plane as that having the optical circuit formed therein, i.e., the direction that is lateral to the plane having the optical circuit formed therein.
For example, a spot size convertor (SSC) having a wire optical waveguide with an apex thereof formed into a tapered shape or a reverse tapered shape is most ordinarily used, and variations thereof have been proposed (see, for example, Non Patent Literature 1 and Non Patent Literature 2). The principle thereof is as follows. The spot size of the propagated light is enlarged with the first core material in the form of a tapered shape or a reverse tapered shape, and the light is propagated through the second core having a larger size than the first core and having a covered apex, and optically coupled with a fiber or the like. The refractive indices of the materials have the relationship, (first core material)>(second core material)>(clad material). However, the structure is limited to be formed in a single plane due to the manufacturing process reasons, and the direction for input and output of light to an optical circuit using the SSC is limited to the lateral direction.
In input and output of light between the optical circuit having the silicon-based wire optical waveguide as a major constitutional component and the other optical devices, such as an optical fiber, a light source and a light receiver, however, various technical advantages, such as inspection of the silicon-based wire optical waveguide device in the stage of wafers, and mount of a light source and a light receiver in the perpendicular direction, may be obtained in the case where the optical circuit is coupled with the other devices in directions other than the in-plane direction, particularly the perpendicular direction.
As a method for coupling light in a direction that is different from the in-plane direction of an optical circuit having a silicon-based wire optical waveguide as a major constitutional component, such a method has been known that a plane diffraction grating coupler is formed at an end of the optical waveguide, and an optical device, such as an optical fiber, is coupled in a direction that is slightly tilted from the perpendicular direction (see Non Patent Literature 3 and Non Patent Literature 4).
Furthermore, there is a report of a method in which even in a rib optical waveguide, a tilted mirror is formed at an apex of an end of the optical waveguide, and thereby light is reflected upward (Non Patent Literature 5). Moreover, there is a recent report of a method in which a silicon-based wire optical waveguide itself is three-dimensionally curved upward and coupled with an optical fiber or an optical waveguide (Non Patent Literature 6 and Non Patent Literature 7).
Among these methods, the method of three-dimensionally curving a silicon-based wire optical waveguide itself upward is an excellent method since the method is free of limitation in wavelength band, which occurs in a plane diffraction grating coupler, and is also free of a problem of increased coupling loss due to the space between the end of the optical waveguide and the mirror, which occurs in the mirror reflection type method. For example, as an optical circuit having a silicon-based wire optical waveguide as a major constitutional component as shown in FIG. 5, the end portion, of the silicon-based wire optical waveguide is curved upward to achieve input and output of light from the above. It is impossible to produce the curved structure herein simultaneously with the process for producing the optical circuit, and therefore it is necessary to use a process technique for curving three-dimensionally upward the end portion of the silicon-based wire optical waveguide of the optical circuit having been produced in advance.
In practice, particularly, such a process technique is necessarily developed that is capable of being applied to an optical circuit, in which the constitutional element thereof may be broken when the circuit is subjected to a high temperature process, for example, an optical circuit formed on a circuit board having a metallic line, and an optical circuit board containing a metallic line for driving an active optical device formed on a single substrate.
As a method for curving three-dimensionally a silicon-based wire optical waveguide, such a method has been known that a silicon oxide film is formed by plasma CVD in an upper portion of the silicon-based wire optical waveguide, and the silicon-based wire optical waveguide is curved spontaneously by utilizing the difference in residual stress from the thermally oxidized silicon oxide film as the underlayer of the silicon-based wire optical waveguide (see Non Patent Literature 3).
A method of curving a silicon-based wire optical waveguide itself three-dimensionally upward is excellent as a method for coupling light in a direction that is different from the in-plane direction of the silicon-based wire optical waveguide formed therein.
However, the method, in which a silicon oxide film is formed by plasma CVD in an upper portion of the silicon-based wire optical waveguide, and the silicon-based wire optical waveguide is curved spontaneously by utilizing the difference in residual stress from the silicon oxide film formed by thermal oxidization as the underlayer of the silicon-based wire optical waveguide, has the following problems.
(1) It is necessary to form by dry etching a cantilever beam structure having a silicon-based wire optical waveguide that is held in vertical direction with a silicon oxide film and a thermally oxidized silicon oxide film, which are formed by plasma CVD, and such a complicated process is necessarily performed that after deeply etching the thick silicon oxide film, the upper part of the silicon substrate under the oxide film having the cantilever beam structure is bored.
(2) Due to the principle utilizing the difference in residual stress, the curvature is constant over the entire cantilever beam structure, and it is difficult to change the curvature locally.
(3) For directing the apex of the curved portion to an arbitrary direction, it is necessary to control strictly the length of the cantilever beam, the thicknesses of the upper and lower oxide films, the heating temperature, and the like, and a high processing accuracy is demanded.
(4) Due to the use of the difference in residual stress between the upper and lower oxide films, it is difficult to curve the optical waveguide in the case where the upper and lower oxide films are formed by the same film formation method.
(5) A high temperature heating process is required for providing a large curvature amount, and therefore it is impossible to apply the method to an optical circuit, in which the constitutional element thereof may be broken when the circuit is subjected to a high temperature process, for example, an optical circuit formed on a circuit board having a metallic line, and an optical circuit board containing a metallic line for driving an active optical device formed on a single substrate. Further, cost of the process will increase.
(6) It is considered that the most ideal structure of a three-dimensionally curved waveguide is the aforementioned SSC structure that is entirely curved three-dimensionally or the aforementioned SSC structure, the apex portion of which is three-dimensionally curved, since an SSC has superior performance to a plane diffraction grating coupler except for the point that an SSC is impossible to achieve optical coupling in the perpendicular direction. However, it is impossible in principle to curve by the method of Non Patent Literature 5 the structure containing not only the first core material but the second core material, the third core material, the clad material and the like, which are fabricated complicatedly.
For solving the aforementioned problems, as shown in FIG. 6, such a processing method of a silicon-based wire optical waveguide that an ion beam is implanted in the particular direction from the outside of the silicon-based wire optical waveguide having a cantilever beam structure, and thereby a stress is formed inside the wire structure itself to curve the optical waveguide, is proposed (see Patent Literature 1).
The summary of the aforementioned proposal will be described with reference to FIGS. 6(A) to 6(F).
(1) As shown in FIG. 6(A) and FIG. 6(B), which is a side view thereof, a silicon layer on a silicon oxide film, which is a support layer 2, formed on a silicon substrate 1, which is an optical circuit board, is processed by a lithography technique, so as to forma silicon-based wire optical waveguide 3.
(2) As shown in FIG. 6(C), apart of the support layer 2 is removed to make such a structure that an end portion 4 of the silicon-based wire optical waveguide 3 thus formed is extended into free space in the form of a cantilever beam from a flat end forming the silicon-based wire optical waveguide.
(3) The entire surface of the silicon substrate 1 forming the cantilever beam structure is irradiated with an ion beam in a particular direction above the substrate, as shown in FIG. 6(D), so as to curve upward the end portion 4 of the silicon-based wire optical waveguide having a cantilever beam structure, as shown in FIG. 6(E).
According to the method, the following advantages may be obtained, as compared to the ordinary processing method, in which a silicon-based wire optical waveguide is held with upper and lower oxide films.
(1) The upper and lower oxide films are unnecessary since the internal strain stress of silicon wire itself is utilized.
(2) The curvature may be controlled by controlling the implantation direction and the irradiation amount of the ion beam.
(3) A steep curvature with a curve radius of 5 μm or less may be formed, and thus the device may be miniaturized.
(4) The curved end portion may be extended in a self-aligning manner in the ion implantation direction.
(5) The method may be performed by a low temperature process, and thus may be applied to an optical circuit that is broken by a high temperature process.
(6) An ordinary SSC may be curved and then subjected to optical coupling from the upper and lower sides thereof. Furthermore, only by curving the first core material, the second core material may be formed in a subsequent step at the end portion thereof to constitute an SSC.
According to the advantages, an advantageous effect of achieving a processing method with high mass productivity may be obtained.
However, in the aforementioned proposal, the end portion of the silicon-based wire optical waveguide may be curved, but the portion of the silicon optical waveguide that is not intended to be curved is also subjected to ion irradiation with high energy, so as to cause a large optical transmission loss of approximately 60 db/mm, which prevents the method from being applied to practical use.